This invention relates to a method of addressing a matrix of bistable pixels which are defined by areas of overlap between members of a first set of electrodes on one side of a layer of ferroelectric material and members of a second set of electrodes, which cross the members of the first set, on the other side of the material, in which method blanking signals are applied to the members of the first set of electrodes to effect blanking before unipolar select signals are applied thereto one by one to effect writing to the corresponding pixels by simultaneously applying a chosen data waveform to each member of the second set of electrodes, the data waveforms each including a data section coinciding with a select signal, in between a charge-balancing section which charge-balances the data section and a further section.
A known drive scheme for multiplex addressing FLCDs, known as line blanking, is described in GB 2173336, and shown diagrammatically in FIG. 1. The row electrodes of the device are scanned with a "blank" waveform 6 of amplitude Vb, followed by a `select` waveform 3 of amplitude Vs. One of two data waveforms "unchanged" 8 or "on" 10, each of amplitude Vd, is applied to each column electrode simultaneously with the occurrence of each select waveform, and is chosen in accordance with the required state of the pixel in the column which is also in the row having the `select` waveform applied to it. The resultant writing waveforms appearing across the pixel are shown at 12 and 14. The `blank` waveform 6 sets the pixels of the row to a dark state regardless of which data signal it combines with, i.e. whether resultant waveforms 10 or 12 appear across the pixels. When a row is neither being selected nor blanked, i.e. the non-select signal 4 is applied to the row, the resultant waveforms 16 or 18 appear corresponding to the data signals 8, 10 neither of which change the state of the pixels.
This drive scheme is suitable for use in the so called `inverse` mode of operation of the ferroelectric material where the voltage which switches the pixel given a certain pulsewidth is lower than that which leaves it unchanged. However, it is unsuitable for use in the normal mode, where the opposite is true, although operation in this mode is desirable due to the lower drive voltages required.
FIG. 2 shows the switching characteristic, pulsewidth W versus voltage V, of a typical ferroelectric material such as liquid crystal. The part of the characteristic within which switching occurs is denoted as 100 and the part within which switching does not occur is denoted as 101. It can be seen that the curve is much less steep in the normal mode part 102 than in the inverse mode part 103, so that the data voltages Vd must be much larger in order to ensure that the applied pulses fall within the correct part of the switching characteristic even when outside factors such as temperature change cause it to vary. This leads to the problem that the data voltages alone (i.e. combined with a non-select pulse) may be sufficient to cause unwanted switching where data waveforms of opposite senses follow each other and effectively extend the widths of the pulses.
The scheme shown in FIG. 3 has been proposed by T. Numao and M. Koden in a paper "Driving waveforms of partial writing scheme for FLCD" in "Displays" vol. 14 no. 3 at pages 139-143 (July 1993) to alleviate this problem. In this scheme, known as a `three-slot` scheme, the data waveforms "unchanged" 24 and "on" 26 each have three sections. The middle sections, which coincide with the select pulse 28, are of opposite polarities, and the positive and negative parts of each waveform are in the same order so that a pulse of a particular polarity is never followed immediately by another of the same polarity. Although this scheme reduces the risk of unwanted switching, it also slows down the addressing of the matrix since another time period is added to each waveform. The non-select and blank waveforms are denoted by 104 and 105 respectively in FIG. 3.